General kinetic description of multioxide silicate mineral and glass dissolution
Résumé
The dissolution mechanisms of multioxide silicate minerals and glasses differ from those of single (hydr)oxides because their dissolution may require the breaking of more than one metal-oxygen bond type. A general kinetic description of major rock forming multioxide silicate dissolution is developed in the present study by assuming the following: (1) the relative rates at which various metal-oxygen bonds are broken within a multioxide structure are consistent with the relative dissolution rates of the single (hydr)oxides; (2) the difference in the rates of breaking each metal-oxygen bond type is sufficiently large such that the reaction breaking one bond type can attain equilibrium before breaking substantial quantities of slower breaking metal-oxygen bonds; and (3) those metal oxygen bonds that break before the final destruction of the structure liberate metal atoms via metal-proton exchange reactions. Multioxide dissolution proceeds via a series of metal-proton exchange reactions until the mineral or glass structure is destroyed. This metal-proton exchange reaction sequence is shown to be consistent with leached layer compositions at acidic conditions. The last metal-proton exchange reaction in the series is slowest and thus rate controlling. Of these slowest exchanging metals, those partially freed from the structure by being adjacent to previously exchanged metals are liberated faster than those completely attached to the mineral or glass and thus constitute the rate-controlling precursor complex. The identity and reactions forming this precursor complex are used within the context of transition-state theory to derive equations that describe accurately the dissolution rates of the major rock-forming multioxide silicate minerals and glasses as a function of solution composition over the full range of chemical affinity.